The present invention relates to a method of forming a tappet in an internal combustion engine, the outer circumferential surface of a tappet body being covered with a coating layer.
Tappets used in a direct acting type valve operating mechanism of an internal combustion engine are made of Al alloy to lighten it. Al alloy tappets are poorer than Fe alloy tappets in mechanical strength and wear resistance. Therefore, coating layer made of hard material is formed on the outer circumferential surface by thermal spraying which can be relatively easily carried out.
To form the coating layer by thermal spraying, it is necessary to increase adhesion strength of spraying material and peel resistance by making the outer circumferential surface of the tappet rough in a prior step. However, rough surface treatment such as blasting needs a long time which causes low productivity, and a lot of blasting materials are required, thereby increasing manufacturing cost.
To solve the problem, as shown in FIG. 9, a method of forming a tappet is suggested in which a rough surface which comprises a spiral groove 2 is formed on the outer circumferential surface of a tappet body 1 by a cutting tool, hard material being thermally sprayed onto the surface to form a coating layer.
However, as mentioned above, in the method wherein the coating layer 3 is formed on the spiral groove 2, when the outer circumferential surface is finished after thermal spraying, a chamfered portion 4 is formed at the lower end, so that a crest of the spiral groove 2 is exposed on the coating layer 3, which decreases in thickness at the lower end and is discontinuous, thereby decreasing adhesion strength of the coating layer 3, which is liable to peel off. If small hard peeled pieces come into an interface between a cylinder head and the tappet during operation of the engine, sliding surfaces significantly wears.